SvelteKit (@sveltejs/kit) before 3.0.0 contains an open redirect vulnerability in enhanced form actions (use:enhance) on routes accepting paths beginning with a double slash. Attackers can craft action paths starting with // that become protocol-relative URLs, causing non-redirect form results to navigate victims to attacker-controlled sites.

Affected: npm: @sveltejs/kit

### Impact Applications using enhanced form actions (`use:enhance`) on routes that accept paths beginning with `//`, such as catch-all routes, may be vulnerable to an open redirect.

### Patches A fix has been prepared for `@sveltejs/kit` 3.0.0. Users should upgrade to 3.0.0 or later.

The fix preserves leading double slashes in action paths without generating protocol-relative URLs, restricts non-redirect action navigation to the current origin, and includes the origin when comparing action locations with the current page.

### Workarounds Reject requests whose parsed URL pathname begins with `//` before route resolution, for example in a reverse proxy or a SvelteKit `handle` hook.

Alternatively, disable navigation for non-redirect enhanced form results by supplying a custom submission callback:

```svelte <form method="POST" use:enhance={() => ({ update }) => update({ navigate: false })}> <!-- form fields --> </form> ```

This applies non-redirect results to the current page instead of navigating to the action location.